[30.03] Evolution of the Orbits of Extrasolar Planet Moons During Planet Migration

J. W. Weiss, G. R. Stewart (CU/LASP)

With the discovery of ``hot Jupiters" (Marcy, G. and P.
Butler, 1998, ``Detection of Extrasolar Giant Planets"
Annual Review of Astronomy and Astrophysics, 36), ,
migration of planets due to planet-disk interactions has
become widely accepted as an explaination for these bodies.
Migration of these planets could have serious effects on any
moons that the planets might have. It is clear that if the
planet migrates in so close to its star that a moon is no
longer within the Hill sphere (the region where the planet's
gravity dominates over the star's), the moon will be lost.
However, it is not immediately obvious when and how this
loss occurs.

Using Hill's approximation, we have integrated orbits of
moons as their parent planet migrates inward over a period
of approximately 10,000 years. We find that prograde
orbiting moons drift away from their planets. Retrograde
moons, on the other hand, migrate inwards towards their
planets. In both case, however, the Hill radius shrinks fast
enough to leave the moons outside of the Hill sphere. We
find that when a prograde moon orbits at about 0.5 Hill
radii from the planet, it becomes lost. The corresponding
loss point for retrograde moons is 1 Hill radius, indicating
that retrograde moons will be retained longer. These results
are in agreement with Hamilton and Burns 1991 (Icarus,
92, pp. 118-131).

In particular, for a moon which begins at 0.1 Hill radii
from a Jupiter-like planet (in terms of mass and orbital
semi-major axis), prograde moon loss occurs at roughly 1.3
AU from the parent star, while retrograde moons are lost at
0.1 AU. For prograde moons starting at 0.01 Hill radii from
their planets under the same conditions, loss occurs at
approximately 0.1 AU. We have not seen retrograde moons at
this distance being lost in our simulations. For comparison,
the Galilean moons all orbit between 0.008 and 0.03 Hill
radii from Jupiter at present.

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